This invention relates generally to systems for providing imagery in the visible portion of the optical spectrum from thermal energy radiated from within a field of view.
Thermal imaging systems with detector and light emitting diodes (LED) arrays operatively associated with a single scan generator are known; and one such system is described in U.S. Pat. No. 3,626,091. This type of thermal imaging system, sometimes referred to as the "parallel-scan" type of system, is quite suitable for many applications. However, as performance requirements, e.g., range, resolution and field of view, are increased certain problems resulting from the fact that in such systems different detector elements exclusively view associated portions of the field of view, and that a relatively large number of detectors are required to scan the field of view with a "low-order" interlaced scan pattern. For example, the systems signal to noise ratio (S/N), field of view and detector configuration are so interrelated that design flexibility is restricted. Further, since each detector must drive a separate signal processing channel and many detectors are required, the addition of desirable but sophisticated signal processing techniques can become quite costly since they must be reproduced in many channels. Such signal processing techniques might include, for example, flare suppression circuits, DC restoration circuits; and variable aperture correction means. Further, since a primary requirement for good viewability is response uniformity, matching the responsivity of the various channels on a continuous basis and keeping the number of "dead end" channels to a minimum, is highly desirable with parallel-scan type systems. This restriction on the tolerable number of "dead channels" imposes a severe yield problem on detector manufacture as well as a problem in system maintainability. Yet another disadvantage of parallel-scan systems is that efficient detector cold shielding is difficult to achieve because of the large field of view subtended by the large detector array normally associated with such systems.
Many of the above described problems were resolved by the improved mechanization described in U.S. Pat. No. 3,723,642 which allows high performance levels to be reached with a drastic reduction in cost. In accordance with the just cited patent, a field of view is optically scanned in a two dimensional pattern by each element of a linear detector array. Output signals from each detector element are delayed as a function of the scan rate and the relative position of the element in the array to allow for the summation of signals from the same image segment provided by the various elements of the array. The approach of the just cited patent provides system simplicity relative to the parallel-scan technique; however, electronic processing is required so that the output from each of the detector elements may be superimposed and a single output signal suitable for processing by remote electronic type display unit is provided.